DNa, GeNes, aND BioteChNoloGy 415What is recombiNaNt DNa?- Recombinant DNA combines the DNA from different species.
- Recombinant DNA can be inserted into rapidly dividing cells
that can make multiple, identical copies of it for use in genetic
engineering. - PCR is a rapid method of amplifying DNA in test tubes.
taKe-home messaGeF A collection of
recombinant plasmids
containing foreign DNA.A A selected restriction enzyme
cuts wherever a specific base
sequence occurs in a molecule
of chromosomal DNA or cDNA.
B The same enzyme cuts the
same sequence in plasmid DNA.C DNA or cDNA
fragments with
sticky ends.E The
foreign DNA,
the plasmid
DNA, and
modification
enzymes
are mixed
together.G Host cells
able to divide
rapidly take up
recombinant
plasmids.D Plasmid DNA
with sticky ends.DNA clone because when a bacterium copies a plasmid, it
makes many identical, “cloned” copies of it.
A DNA clone carries foreign DNA into a host cell that
can divide rapidly (such as a bacterium or a yeast cell).
This can be the start of a cloning “factory”—a population
of rapidly dividing cloned cells, all with identical copies of
the foreign DNA (Figure 21.13). As they divide they make
much more of, or amplify, the foreign DNA.pCr is a super-fast way to copy Dna
The polymerase chain reaction, or PCR, is an even faster way
to copy DNA. These reactions occur in test tubes, and prim-
ers get them started.
A primer is a manmade, short nucleotide sequence that
base-pairs with any complementary sequences in DNA. The
workhorses of DNA replication—the DNA polymerases—
chemically recognize primers as start tags. Following a
computer program, machines make a primer one step at
a time.
PCR also uses a DNA polymerase that is not destroyed
at the high temperatures that are required to unwind a
DNA double helix. (Such high temperatures will denature
and destroy the activity of most DNA polymerases.)
In the lab, primers, the polymerase, DNA from an
organism, and nucleotides are all mixed together. Next, the
mixture is exposed to precise temperature cycles. During
each temperature cycle, the two strands of all the DNA
molecules in the mixture unwind from each other.
Primers line up on exposed nucleotides at the targeted
site according to base-pairing rules (Figure 21.14). Each
round of reactions doubles the number of DNA molecules
amplified from the target site. For example, if there are 10
such molecules in the test tube, there soon will be 20, then
40, 80, 160, 320, and so on. Very soon there will be billions
of copies of a target piece of DNA.
PCR can amplify samples that contain tiny amounts of
DNA, and it is used in laboratories all over the world. As
you’ll read shortly, it can copy DNA from even a single hair
follicle or a drop of blood left at a crime scene.F i g u r e 21.14 Animated! PCR makes quick work of copying
DNA. The photograph shows rows of PCR systems that are
copying human DNA. Right: The steps of the polymerase chain
reaction (PCR). (© Cengage Learning)Volker Stege420r/SPL/
Science SourceA DNA (blue) is
mixed with primers
(red), nucleotides,
and heat-tolerant
DNA polymerase.B When the mixture
is heated, DNA strands
separate. When it is
cooled, some primers
bond to the template
DNA.C DNA polymerase
uses the primers to
begin synthesis, and
complementary strands
of DNA form. The first
round of PCR is now
complete.Each round can
double the number
of DNA molecules.
After thirty rounds, the
mixture contains huge
numbers of DNA
fragments, all copies
of the starting DNA.E DNA polymerase
uses the primers to
begin DNA synthesis,
and complementary
strands of DNA form.
The second round
of PCR is complete.D The mixture is heated
again, and all of the
DNA separates into
single strands. When
the mixture is cooled,
some of the primers
bond to the DNA.Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).